In order to meet requirements for operating lifetime and reliability as needed by very long distance transmission links, it has become necessary to provide a degree of redundancy in optical fiber transmission systems, in particular concerning the laser diodes which are fitted in the repeaters. This redundancy is obtained by initially equipping each repeater with several spare laser diodes which are brought into operation one by one by a switching circuit. Such a configuration requires means for remotely controlling the repeater switching circuits from the line's terminal equipment, together with remote signalling means from each repeater for informing the terminal equipment on the state of the laser diodes in service.
U.S. Pat. No. 4,281,416 describes an optical fiber digital link fitted with remote control enabling the laser diode in service in each repeater to be replaced by a spare laser diode, together with remote signalling specific to each repeater enabling an alarm to be transmitted when the bias current of the laser diode in service exceeds a threshold. The remote control signal is constituted by a digital message which is of the same nature as the digital signal transmitted over the link, and which is inserted into the transmitted traffic by the remote surveillance terminal. This digital message is repeated consecutively several times in order to avoid spurious operation, and it is constituted by a portion which identifies the repeater concerned, and a portion which identifies which laser diode is to be put into service in that repeater. The remote signalling signal is a pattern of the same nature as the transmitted digital signal, it is a signal which replaces traffic, and it is constituted by a pseudo-random binary string whose initial value identifies the source repeater.
This remote signalling system has the drawback of requiring each repeater to be fitted with a source of pseudo-random strings which is capable of operating at the digital signal bit rate which, in an optical fiber digital transmission system, is an extremely high rate and must be provided using ECL technology and therefore consumes a great deal of energy. This constitutes a significant increase in the total energy consumed by the repeater, and above all it constitutes a considerable increase in the heat which the repeater must be capable of dissipating.
The above-described prior art system also suffers from the drawback of interrupting the link for a period of time which is long enough to cause synchronization to be lost in the digital data stream hierarchy, which means that such a signalling system can only be used for transmitting major alarms.
Preferred implementations of the present invention avoid these drawbacks and provide remote signalling using a message which momentarily replaces the data stream conveyed by the link, which requires a minimum of very fast circuits for generating the message, and which do not disturb the digital data stream and the processing thereof other than at the instants at which the message is being transmitted.